Gas igniter for pulverized coal burners



Jan. 15, 1957 2 Sheets-Sheet 1 Filed July 14, 1952 Ka li lNvENi'oRs ione]! E Jb/mson BY George Musaz ATTORNEY Jan. 15, 1957 JOHNSON ETAL2,777,512

GAS IGNITER FOR PULVERIZED COAL BURNERS Filed July 14. 1952 2Sheets-Sheet 2 No 8, mm 5 w w R N 2 h R Q Q 0 ATTORNEY United StatesPatent GAS IGNITER FOR PULVERIZED COAL BURNERS Lowell E. Johnson,Alliance, Ohio, and George Musat, East Elmhurst, N. Y., assignors to TheBabcock & Wllcox Company, New York, N. Y., a corporation of New JerseyApplication July 14, 1952, Serial No. 298,674

7 Claims. (Cl. 158-115) This invention relates to igniters for fuelburners and,

more particularly, to a reliable, relatively simple, and easily servicedgas igniter particularly useful with pulverized coal burners.

In the burning of pulverized fuel, such as pulverized coal, or a mixtureof air and the pulverized fuel is delivered to the furnace where it isadmixed with additional, or secondary air, for complete combustion.Reliable means must be provided for insuring immediate ignition andmaintenance of ignition of the pulverized fuel and air mixture directlyafter introduction into the furnace, particularly so that the furnacewill not become filled with an unburned combustible and explosivemixture. Supplementary maintenance of ignition is of particularimportance when the mixture of pulverized coal and air delivered to thefurnace has a relatively high humidity or moisture content, which maymake it difficult for the combustible mixture to maintain its ownignition up to the time when the furnace is heated to a relatively highoperating temperature.

For this reason, various types of igniters are associated withpulverized fuel burners, utilizing such relative easily ignited fuels asoil and gas. The choice of fuel for the igniter is frequently dependentupon the availability of a particular fuel. Where large supplies of gasare easily available, for example, the igniters are generally of the gasburning type.

There are certain prerequisite characteristics necessary to provide asatisfactory igniter. Among other desirable characteristics, the ignitermust be reliable in its ignition function, so that there is assurance ofthere being an ignition flame each time the igniter is turned on. Theigniter flame must be large enough to ignite the stream of airbornepulverized coal or other fuel smoothly and the igniter flame must becompletely stable. In addition, it is desirable that the igniter becapable of extended periods of operation, in order to maintain ignitionof pulverized fuel and air mixtures of high humidity or moisture contentwhen the furnace has not been heated to a high operating temperature, orwhen the furnace is operating at low load wherein loss of coal ignitionis a distinct possibility. Finally, the igniter must be so constructedthat it can be easily maintained and serviced without requiring theemployment of specialized personnel.

Some of the foregoing requirements conflict with others, so that it isrelatively diflicult to provide an igniter fully meeting all of theserequirements. For example, a large flame for lighting the fuel requiresa correspondingly high volume of gas supplied to the lighter. In turn,if the gas volume is relatively large compared to the availablecombustion air at the ignition zone of the lighter, it is diflicult tomaintain the igniter flame burning stably. Correspondingly, if only thetheoretically correct proportions of gas and air are supplied at theignition zone of the igniter, then only a relatively small stable flamecan be produced.

7 Such igniters are generally started by a spark between two ignitionelectrodes having their sparking ends adja- 2,777,512 Patented Jan. 15,1957.

cent the zone where the gas is admixed with air; A high potential isapplied to these electrodes, requiring a correspondingly good insulationof the electrodes. Frequently, due to the necessity of maintaining thesparking electrodes adjacent the burning gas flame, the insulation ofthe electrodes may be seriously deteriorated resulting in shorting ofthe electrodes. Attempts to avoid these difllculties have producedelectrode assemblies so arranged relative to the issuing combustiblemixture that it is difficult to remove, inspect, and/or repair theelectrode assembly.

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In accordance with the present invention, a gas igniter is provided inwhich the aforementioned desirable characteristics are obtained whilestill providing for easy inspection and maintenance of the igniter andparticularly the electrode assembly. To this end, the gas supply isintroduced through a relatively elongated conduit having a meteringorifice therein and arranged to deliver the gas into the outer end of anelongated flame retainer or shield having perforations for admission ofcombustible air to mix with the gas. The electrode assembly is arrangedin parallel spaced relation with the gas supply conduit with the arcingends of the electrodes thus offset laterally from the axis of the gasconduit. Due to this parallel arrangement of a relatively straightelectrode assembly, the latter may be easily withdrawn through the outerend of the igniter assembly for inspection and repair. The electrode.assembly includes supportinginsulator means for the live electrodeswhich are shielded within a tubular member so that insulators are notonly adequately supported but also protected from the gas flame.

A feature of the invention is that the supply of raw gas to the igniterflows through two branches, one branch being the aforementioned gassupply conduit. Upstream from the metering orifice in such conduit, alarge diameter raw gas pipe is branched oif and arranged to extendthrough the flame retainer to the outer end of the latter,

considerably in advance of the end of the electrode assembly where theinitial pilot flame is provided. By this arrangement, the relativelysmall pilot flame is stably maintained in a shielded position where itis protected from the inrush of air for the pulverized fuel burners.This relatively small pilot flame furnishes a continual source ofignition for the raw gas delivered from the furnace or inner end of theflame retainer. The combustion air adjacent the furnace end of theigniter supports combustion of the igniter raw gas to provide a longsoft flame for maintaining ignition of the pulverized fuel. Should thislong flame go out, it is immediately relighted from the constantlyburning shielded pilot flame.

For an understanding of the invention principles, reference is made tothe following description of a typical embodimentthereof as illustratedin the accompanying drawings. In the drawings:

Fig. l is a partial sectional view of a pulverized fuel fired furnaceshowing the invention igniter as arranged to extend between roof tubesof the furnace into igniting relation with a combustible mixture ofpulverized fuel and air delivered between such roof tubes;

Fig. 2 is the longitudinal section View through a gas igniter embodyingthe invention;

Figs. 3 and 4 are elevation views, respectively of th outer and furnaceends of the igniter;

Fig. 5 is an axial sectional view through the gas conduit and itsmetering orifice, taken on the line 5-5 of Fig. 2; and

Fig. 6 is a diametric sectional view on the line 6-6 of Fig. 5,illustrating an interchangeable metering orifice member.

Referring first to Fig. 1 of the drawings, the igniter 10 of theinvention is shown as arranged in operative relation in a pulverizedfuel fired combustion chamber 11 having roof tubes 12. Igniter 10 issuitablysuppgrtedin the front casing 13 of the windbox 14.

Furnace or combustion chamber 11 is supplied with pulverized fuel andprimary air by a burner arrangement comprising, in the usual case, aplurality of primary air and coal delivery nozzles extending throughwindbox .14 and between the roof tubes 12 lining the refractory roof 16of chamber 11, secondary air being introduced into the furnace fromWindbox 14 which is supplied with air under pressure in the usualmanner. A typical pulverized fuel burner arrangement of this type isshown and described in U. S. Patent No. 2,397,793, issued to-L. L. Leachon April 2, 1946.

To simplify the present disclosure, only a single primary air and coal.delivery nozzle 15 is shown extending through windbox 14 and between theroof tubes 12 and into furnace 11,it being understood that more than onecoal delivery nozzle 15 may be operatively associated with igniter 10.

In the particular arrangement shown, igniter 10 extends downwardly andinwardly into furnace 11 so that the flame delivered axially from theigniter will intersect the axis of nozzle 15. The igniter is in aseparate compartment in the windbox provided by shielding housingmembers 17 and 18 which divide the igniter compartment 20 from the mainpart of windbox 14. Refractory bricks 19 close the space between nozzle15 and member 18 to prevent direct impingement of the secondary airunder pressure, from windbox 14, onto the ignition flame directed fromthe outer end of igniter 10 across the primary air and coal streams fromnozzle 15. Housing member 17 has an aperture 21 therein by means ofwhich the secondary air under pressure in windbox 14 flows into andthrough compartment 20 and thence into furnace or chamber 11, enteringthe latter between roof tubes 12. The secondary air flowing throughcompartment 20 supports combustion in the igniter. The usual studs areremoved from roof tubes 12, throughout the portions thereof at the openend of compartment 20, to provide space for igniter 10 and for flow ofsecondary air into chamber 11.

Igniter 10 has its elements generally supported from a relatively largeflat plate 24 bolted or otherwise secured to casing 13 and forming asealing closure for an aperture 26 in the casing section, and throughwhich the igniter is inserted into the compartment 20. A second plate 27is secured to extend perpendicularly to plate 24 near the lower edge ofthe latter, this plate being very substantially narrower than plate 24,as best seen in Fig. 3. A third, or face, plate for the igniter isarranged to extend at an angle to both plates 24 and 27, being welded tothe latter at its upper and lower edges. Triangular gussetsi28, weldedto all three plates, enclose the space between plate 25 and plates 24and 27. Face plate 25 has an upper relatively small circular aperture 31therein through which is tightly fitted the main gas conduit 30. Asubstantially larger aperture 32, below aperture 31, serves forinsertion and removal of the ignition electrode assembly generallyindicated at 35.

The operating or outer end of the igniter is normally covered by atightly-fltting enclosing housing 33 which slips over plate 27 and has afairly close fit withgussets 28. The latter are tapped to receive studs34 by means of which housing 33 is removably held in place. Thesidewalls of housing '33 have slots 36 fitting over. an elbow portion 37 ofconduit-.30 and slots 39 receiving the studs 34. Access to the outer endof the electrode assembly 35 is attained by merely loosening studs 34and then sliding cover 33 rearwardly.

Conduit extends substantially at right angles to plate25 into ignitercompartment 20, and the inner-end of conduit 30 has a flange 38 weldedthereto. Flange 138 is apcrtured to receive bolt and nut assemblies. 41,41 by means of which flange 38 is secured to, a similar flange 42 on;aninner gas conduitsection 40. :Disposedbetween flanges 38 and 42, andeach engaging a flange, are suitable h a resistantssslset .4 cash, ha ina Per p a s us with those of collars 38 and 42. Gaskets 43 serve toefiect a tight seal with a metering orifice member 45 arranged betweenthe gaskets and having a peripheral contour identical with those of thecollars and gaskets, as best seen in Figs. 5 and 6. Metering orificemember 45 is one of severalzinterchangeable metering plates, each havinga different sized orifice46 therein by means of which therateof gas flowthrough conduits 30 and may be set at any desired value. To afford easyinterchangeability of the meteringorifice members, each one thereof hasa pair of open ended slots 47, 47 which slip over bolts 41. Thereby, themetering orifice members may be easily changed merely-'by loosening thebolt and nut assemblies and removing one member and replacing it withanother member.

The inner end of conduit section 40 is threaded into a pipe coupling 48having its other end closed by a threaded plug 51. drilled axially toprovide a gas discharge passage 52. Coupling 48 fits within an aperturein a rectangular plate 50 serving as the outer wall of a flame retainergenerally indicated at 55. In the particular embodiment shown in the.drawings, flame retainer is rectangular in cross section, beingrelatively narrow and high, as seen in Fig. 4, so asto fit readilybetween roof tubes 12. If necessary, the latter may be spread slightlyto receive the flame retainer. The inner end of the flame retainer isbeveled, as at 56, so that the plane of this end extends at only aslight angle to the axes of tubes 12 (Fig. l).

A second plate 57 is spaced somewhat inwardly from plate 50 and has anaperture 58 coaxial with a nozzle 60, secured to plate 57 and extendinginwardly therefrom in axial alignment with gas conduit section 40.Between plates 50 and 57,.the sidewalls or retainer 55 are formed withrelatively large apertures 61. Plate 57 is Welded at its lower edge tothe bottom wall of retainer 55 and its upper edge is welded to anelongated plate 62 forming, with the upper wall and side walls of theretainer, a raw gas passage extending completely throughout the lengthof the retainer 55. Apertures 61 provide for air to flow into the mixingcompartment 65 formed by plates 50, 57, 62 and the lower and side wallsof flame retainer 55.

Intermediate the ends of conduit section 30, a raw gas conduit is joinedthereto, as by welding, and has its inner end threaded into a reducer 71having its smaller end welded into an aperture in plate 50. By thismeans, raw gas flows, without the restriction of a metering orifice,from conduit section 30 into the raw gas passage 72, branching fromsection 30, for discharge from the outer end of igniter 10.

Electrode assembly 35 includes a pipe section 74 having its outer endseated in an aperture in a flange 75 disengageably secured in air tightrelation to plate 25 by wing nuts 73. The inner end of pipe 74 isthreaded into a coupling 76 into which is threaded a second pipe section77 threaded, at its inner end, into a coupling 78. A third pipesection81 is threaded into the inner end of coupling 78, and the innermost endof pipe section 81 is cut away on a taper, as indicated at 82, toprovide a projecting strip 83. Grounded electrode is in the form of astud threaded into the outer end of section 83 and held in place by alock nut 84. Pipe section 81 extends into the 'flame retainer 55 throughan aperture 86 in plate 50.

The live electrode is substantially a rectilinear rod having its innerextremity disposed in adjacent spaced relation to electrode 80.Electrode 85 is mounted through a pair of insulators 87, 88 which aresupported in rings 91 mounted respectively in the inner end of pipesection 74, both ends of, pipe section 77, and the outer end of pipesection 81. Between each pair of supporting rings, packt ing 92 isdisposed around the insulators 87 and 88. Where electrode 85 emergesfrom, insulator 88,.apair-of short pieces of steel 93," 93 are welded totheelectrode and act as a locating means for the electrode in theinsulators. The outer end of electrode 85 is threaded to receive nuts 94positioning a washer 96 engaging a spring 97 coiled about the electrodeand seated against a washer 98 on the outer end of insulator 87. Hightension current is supplied to electrode 85 by a conductor 95 having aterminal secured to the electrode by a nut 99. The spring 97 permitsrelative expansionot electrode 85 without injury to insulators 87 and88, while still maintaining the electrode assembly in its operatingrelationship.

Aperture 32 in plate 25 is somewhat larger than couplings 75 and 78.Thereby, the electrode assembly may be readily withdrawn for inspectionand repair by removing nuts '73, after cover 33-has been removed, andsimply drawing assembly 35 outwardly through aperture 32. The use of therectilinear electrode 85 enables relatively easy and simple seating ofthe spark gap between the inner end of this electrode and electrode 80without the complications involved where bent electrodes are used. Itwill be noted that the electrode assembly is in parallel spaced relationwith the gas supply conduits 30 and 40, and discharge nozzle 60, thespark gap being located somewhat in advance of the discharge nozzle andat the optimum location for ignition of the gas mixture.

In operation, the size of orifice 46 is so selected that the gas-airmixture discharged from nozzle 60 is at the proper ratio for readyignition by the spark from the electrodes. This relatively small, orpilot flame, is readily ignited and readily maintained, being protectedfrom the rush of air through compartment 20 by the perforated flameretainer 55. The pilot flame is sufiiciently long, however, to maintainignition of the relatively larger vol ume of raw gas discharged from rawgas nozzle 72 at the outer end of flame retainer 55. The burning rawgas, which is kept constantly ignited by the pilot flame within thefiame retainer, forms a long flame which is fully adequate to reliablyignite the pulverized fuel and primary air mixture discharged fromnozzles 15, and to maintain this ignition even though the fuel-airmixture may have a relatively high humidity or moisture content, or thepulverized fuel burners may be operating at a greatly reduced load.

Due to the setting of the sparking end of the electrode assemblyintermediate the length of flame retainer 55, the hot gases ofcombustion of the flame issuing from nozzle 60 recirculate around theend of the electrode assembly. This heated gas at the ignition pointmakes it relatively easy to maintain ignition. However, the inner end ofthe electrode assembly is set far enough back from the exit end of theflame retainer that the electrodes are protected from any slag beingdeposited thereon from the burning fuel in the combustion chamber orfurnace 11.

While a specific embodiment of the invention has been shown anddescribed in detail to illustrate the application of the inventionprinciples, it will be understood that the invention may be embodiedotherwise without departing from such principles.

What is claimedis:

l. A gas igniter constructed and arranged for mounting in operativerelation to furnace main fuel burners, said igniter comprising a tubularflame retainer having an outer end and a discharge end; means, includingwalls of said retainer and an elongated partition therein, forming a rawgas passage extending longitudinally through said retainer; a gas supplyconduit extending into said retainer through the outer end thereof injuxtaposition with said passage, with its gas discharge axis in spacedsubstantially parallel relation to that of said passage; mixing meanscommunicating with the discharge end of said conduit and arranged toadmix air with the gas discharged therefrom, and including a nozzlehaving its discharge axis in spaced substantially parallel relation tothat of said passage and its discharge end substantially upstream of thedischarge end of said retainer; tubular shielding means extendingthrough the outer end of said retainer in substantially paralleljuxtaposed relation to said supply conduit; a spark-creating electrodeassembly including a relatively elongated substantially rectilinearelectrode removably supported in insulated relation through saidshielding means to extend in substantially parallel spaced relation tosaid supply conduit, said shielding means constituting a cooperatingelectrode; the sparking end of said electrode assembly beinglongitudinally intermediate the discharge ends of said nozzle and saidretainer; and a raw gas conduit connected to said raw gas passage.

2. A gas igniter constructed and arranged for mounting in operativerelation to furnace main fuel burners, said igniter comprising a tubularflame retainer having an outer end and a discharge end; means, includingwalls of said retainer and an elongated partition therein, forming a rawgas passage extending longitudinally through said retainer; a gas supplyconduit extending into said retainer through the outer end thereof injuxtaposition with said passage, with its gas discharge axis in spacedsubstantially parallel relation to that of said passage; 'mixing meanscommunicating with the discharge end of said conduit and arranged toadmix air with the gas discharged therefrom, and including a nozzlehaving its discharge axis in spaced substantially parallel relation tothat of said passage and its discharge end substantially upstream of thedischarge end of said retainer; tubular shielding means extendingthrough the outer end of said retainer in substantially paralleljuxtaposed relation to said supply conduit; a spark-creating electrodeassembly including a relatively elongated substantially rectilinearelectrode removably supported in insulated relation through saidshielding means to extend in substantially parallel spaced relation tosaid supply conduit, said shielding means constituting a cooperatingelectrode; the sparking end of said electrode assembly beinglongitudinally intermediate the discharge ends of said nozzle and saidretainer; and a raw gas conduit branching from said supply conduitupstream of the discharge end thereof and connected to said raw gaspassage.

3. A gas igniter constructed and arranged for mounting in operativerelation to furnace main fuel burners, said igniter comprising a tubularflame retainer having an outer end and a discharge end; means, includingwalls of said retainer and an elongated partition therein, forming a rawgas passage extending longitudinally through said retainer; a gas supplyconduit extending into said retainer through the outer end thereof injuxtaposition with said passage, with its gas discharge axis in spacedsubstantially parallel relation to that of said passage; mixing meanscommunicating the discharge end of said conduit and arranged to admixair with the gas discharged therefrom, and including a nozzle having itsdischarge axis in spaced substantially parallel relation to that of saidpassage and its discharge end substantially upstream of the dischargeend of said retainer; tubular shielding means extending through theouter end of said retainer in substantially parallel juxtaposed relationto said supply conduit; a spark-creating electrode assembly including arelatively elongated substantially rectilinear electrode removablysupported in insulated relation through said shielding means to extendin substantially parallel spaced relation to said supply conduit, saidshielding means constituting a cooperating electrode; the sparking endof said electrode assembly being longitudinally intermediate thedischarge ends of said nozzle and said retainer; flow restrictingorifice means in said supply conduit upstream of said mixing means; anda raw gas conduit branching from said supply conduit upstream of saidflow restricting orifice means and connected to said raw gas passage.

4. A gas igniter as claimed in claim 1 in which said flow restrictingorifice means comprises an orifice plate interchangeably mounted betweendisengageably coupled sections of said supply conduit.

5. A gas igniter as claimed in claim 1 in which the walls of said flameretainer are formed with air admitting apertures including at least onerelatively large aperture Substantially adjacent said mixing means.

6. A gas igniter constructed and arranged for mounting in operativerelation to furnace main fuel burners, said igniter comprising rawgasconduit means, a supply conduit, and tubular shielding means mountedin substantially parallel juxtaposed relation; a relatively elongatedtubular flame retainer assembled with and embracing said conduit means,conduit, and shielding means; mixing means mounted Within said flameretainer downstream of the discharge end of said conduit to admix airwith gas discharged from said conduit, and including a discharge nozzlemounted laterallyadjacent said raw gas conduit means and with its axissubstantially parallel to the axis thereof, the discharge end of saidnozzle being upstream of the discharge end of said conduit means todischarge the mixture in substantially parallel spaced relation to saidraw gas conduit means and upstream of the discharge end of the latter;and a sparkcreating electrode assembly including a relatively elongatedsubstantially rectilinear electrode removably supported in'insulatedrelation through said shielding means to extend in substantiallyparallel spaced relation to said supply conduit, said shielding meansconstituting a cooperating 'electrode; thesparking end of said electrodeassembly being longitudinally intermediate the discharge ends of saidnozzle and said raw gas conduit means.

7. A gas igniter constructed and arranged for mounting in operativerelation to furnace main fuel burners, said igniter comprising raw gasconduit means, a supply conduit, and tubular shielding means mounted insubstantially parallel juxtaposed relation with said conduit laterallyintermediate said conduit means and said shielding means; a tubularflame retainer assembled with and embracing said conduit means, conduit,and shielding means, the discharge end of said conduit being within saidflame retainer; mixing means mounted Within said flame retainerdownstream of the discharge end of said conduit toadmix air with'gasdischarged from said conduit, and including a discharge nozzle mountedlaterally adjacent said ravv gas conduit means and with its axissubstantially parallel to theaxis thereof, the discharge end of saidno'zzle'being upstream of the discharge end of said conduitmdns todischarge the mixture in substantially parallel spaced relation to saidraw gas conduit means and upstream Qfthedischarge end of the latter; anda sparkcreatingelectrode assembly including a relatively elongatedsubstantially rectilinear electrode removably supported in insulatedrelation through said shielding means to extend in substantiallyparallel spaced relation to said supply conduit, said shielding meansconstituting a cooperating electrode; the sparking end of said electrodeassembly being longitudinally intermediate the discharge ends of saidnozzle and said raw gas conduit means.

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